If you dont use emacs, then in a terminal do:
$ lein figwheel
(... cruft ...)
dev:cljs.user=> (in-ns 'sample-reagent.core)
nil
dev:sample-reagent.core!{:conn 2}=> (load "core")
nil
dev:sample-reagent.core!{:conn 2}=> (swap! app-state assoc :text "a new string...")
{:text "a new string..."}